Of all potential pyromancies you can transpose from the Old Witch Soul, B-Team chose this. They could´ve put in the Great Chaos Fireball or Chaos Firestorm since both already have a connection to Izalith; this is just a basic pyromancy.

"Deals less damage than resins" how so? flame weapon adds a solid 50+1.3x for 90s, the same goes for the resin, only the resin cannot buff infused weapons, so how is flame weapon weaker in this scenario?